1. Field of the Invention
The present invention relates to a liquid chromatograph including a high performance liquid chromatograph, and to a column oven for use therein.
2. Description of the Related Art
A liquid chromatograph performs separation analysis while maintaining the separation column at a constant temperature. In order to hold the column temperature constant, the separation column is placed in a column oven. The column oven includes a temperature regulator for maintaining the column oven at a constant temperature. The temperature regulator includes, for example, a block heater as a heat source, and employs an air circulation method for circulating its heat in the column.
The mobile phase of a liquid chromatograph may be a volatile solvent such as methanol, acetonitrile, or isopropyl alcohol (IPA). The separation column is connected to the analysis channel by means of a joint in the column oven, and the mobile phase may leak out through this connection portion. When the mobile phase includes a volatile solvent, vaporized gas is generated from the mobile phase leaked. If that vaporized gas is flammable, that vaporized gas has an explosive limit. Therefore, a gas sensor is provided in the oven so that the vaporized gas concentration in the column oven will not exceed the explosive limit, and monitoring is performed so that the indication by that gas sensor will not exceed a vaporization limit.
Monitoring is performed in such a way that the voltage value output from the gas sensor is read; a leak sensor value is obtained from the voltage value; and a leakage threshold is set for the leak sensor value. In order to leave some room, the leakage threshold is usually set to, for example, 1/10 or 1/20 of the explosive limit. If the leak sensor value exceeds the leakage threshold, an alarm is raised, or the operation of the liquid chromatograph is terminated, to prevent explosion.
Meanwhile, the concentration of an explosive limit depends on the solvent. For example, such concentration of ethanol is about 43000 ppm, while that of acetonitrile is about 30000 ppm. Thus, use of different leakage thresholds for a gas sensor according to the solvent has already been in practice (see JP 2002-267644 A: Document 1). In a method of Document 1, a plurality of solvents are each injected into a column oven until the concentration reaches a concentration that corresponds to the threshold; a value detected by the gas sensor is individually measured for each of the solvents; and each value is set as the threshold. The method of Document 1 presumes that sensitivity characteristics of the gas sensor change due to aging, and therefore solvents are injected into the oven at each time of use of each column oven, and the threshold is directly measured for each solvent in order to calibrate the sensitivity change due to aging.
It has been noticed that a system that monitors a leak sensor value based on an output of the gas sensor, and which detects leakage of a mobile phase solvent by comparing the leak sensor value with a leakage threshold as described above poses a problem in that a prolonged continuous operation of a column oven may cause a false detection of liquid leakage even when no leakage of the mobile phase solvent is present.
One cause of such problem is temporal change in the sensitivity of the gas sensor. FIG. 9 illustrates an example of temporal change in a leak sensor value (described later herein) with no liquid leakage of the mobile phase solvent. The horizontal axis represents a continuous measurement from 18:59:07 to 3:04:54 next day. As can be seen from this figure, the leak sensor value, which is based on an output from the gas sensor, increases gradually over time even when no liquid leakage of the mobile phase solvent is present. Thus, if the leakage threshold is set, for example, to 10000, occurrence of liquid leakage will be detected even when no liquid leakage is present. Moreover, since the sensitivity of a gas sensor also depends on temperature and humidity, a temporal change in the temperature and/or humidity around the gas sensor may cause the leak sensor value to vary, which may also cause a false detection of liquid leakage.